Mark no such host error as downstream (#1089)

Author: iwysiu

experimental/errorsource/error_source_middleware.go

@@ -2,6 +2,7 @@ package errorsource
 
 import (
 	"errors"
+	"net"
 	"net/http"
 	"syscall"
 
@@ -28,6 +29,10 @@ func RoundTripper(_ httpclient.Options, next http.RoundTripper) http.RoundTrippe
 		if errors.Is(err, syscall.ECONNREFUSED) {
 			return res, Error{source: backend.ErrorSourceDownstream, err: err}
 		}
+		var dnsError *net.DNSError
+		if errors.As(err, &dnsError) && dnsError.IsNotFound {
+			return res, Error{source: backend.ErrorSourceDownstream, err: err}
+		}
 		return res, err
 	})
 }